This invention relates to direction finding systems.
When designing HF direction finding (DF) systems for shipborne or mobile applications, the prime restriction is the antenna size which must be necessarily small as compared with the received wavelength. However, a narrow aperture DF system has certain inherent drawbacks:
(1) Field strength sensitivity is poor, since only a small volume of the radiated space energy can be sampled.
(2) Reflections from nearby conducting bodies, particularly those at resonance, produce local distortions of the electromagnetic field which may extend over a radius of several wavelengths. A narrow aperture antenna can only measure the average field vector over its own base. Hence bearing errors will be shown depending on the frequency and the relative positions of the re-radiating bodies with respect to the antenna.
(3) Near-field components of the re-radiated field may be picked up by the antenna elements through direct electrostatic or electromagnetic coupling, the results combining with the far field effects of item (2) above.
(4) In contrast to the local re-radiations with errors which are constant for a given site, there are time-variable errors due to ionospheric reflections. The combined field vectors of the ground wave and the sky-wave produce wavefront corrugations which may be drifting slowly across the base of the antenna. Bearing angle displayed by a narrow aperture system will show periodic oscillations about its mean value.
(5) Polarization tilts of the electromagnetic wave array may also be a source of errors if the antenna elements are allowed to pick up the horizontal component of the field.